Synthesis, single crystal structures and efficient catalysis for tetralin oxidation of two novel complexes of Cu(II) with 2-aminomethyl pyridine

Article abstract of DOI:10.1016/j.molcata.2010.10.014

Two novel Cu(II) complexes were synthesized through the reaction of 2-aminomethyl pyridine (AMP) with CuCl₂·2H₂O by changing the metal/ligand ratio. Their structures were thoroughly characterized by FT-IR, elemental analysis and X-ray diffraction method. The results revealed that complex 1 [Cu(AMP)Cl₂] consists of isolated binuclear molecules unit and displays distorted tetragonal pyramid. Complex 2 [Cu(AMP) ₂(H₂O)₂]Cl₂ exhibits a octahedral geometry. The complexes were both evaluated as catalysts in the tetralin oxidation with TBHP as oxidant. Complex 1 showed high catalytic activity and selectivity towards α-tetralone under mild conditions. Thus, under the optimized conditions (acetonitrile 10 ml, catalyst 0.045 mmol, tetralin 4.5 mmol, 65% TBHP 22.5 mmol, T = 50 °C), the conversion of tetralin reached 89% with a selectivity of 71% towards α-tetralone. Compared with complex 1, complex 2 displayed low catalytic activity mainly due to the strong steric hindrance from the two coordinated 2-aminomethyl pyridine molecules.

Full text of DOI:10.1016/j.molcata.2010.10.014

Journal of Molecular Catalysis A: Chemical 333 (2010) 173–179
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Synthesis, single crystal structures and efficient catalysis for tetralin oxidation of
two novel complexes of Cu(II) with 2-aminomethyl pyridine
Chunling Wang, Yuecheng Zhang, Baoguo Yuan, Jiquan Zhao∗
School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 August 2010
Received in revised form 12 October 2010
Accepted 12 October 2010
Available online 12 November 2010
Two novel Cu(II) complexes were synthesized through the reaction of 2-aminomethyl pyridine (AMP)
with CuCl2·2H2O by changing the metal/ligand ratio. Their structures were thoroughly characterized by
FT-IR, elemental analysis and X-ray diffraction method. The results revealed that complex 1 [Cu(AMP)Cl2]
consists of isolated binuclear molecules unit and displays distorted tetragonal pyramid. Complex 2
[Cu(AMP)2(H2O)2]Cl2 exhibits a octahedral geometry. The complexes were both evaluated as catalysts
in the tetralin oxidation with TBHP as oxidant. Complex 1 showed high catalytic activity and selectiv-
ity towards ␣-tetralone under mild conditions. Thus, under the optimized conditions (acetonitrile 10 ml,
catalyst 0.045 mmol, tetralin 4.5 mmol, 65% TBHP 22.5 mmol, T = 50 ◦C), the conversion of tetralin reached
89% with a selectivity of 71% towards ␣-tetralone. Compared with complex 1, complex 2 displayed low
catalytic activity mainly due to the strong steric hindrance from the two coordinated 2-aminomethyl
pyridine molecules.
Keywords:
Tetralin
␣-Tetralone
Oxidation
tert-Butyl hydroperoxide
Cu(II) complex
© 2010 Elsevier B.V. All rights reserved.
[28–34]. To the best of our knowledge, in 1984, Cu2+ was first used
for the oxidation of tetralin [31], however, only a conversion of 27%
1. Introduction
In recent years, a considerable amount of work has been carried
out on the oxidation of organic compounds with transition met-
als as catalysts and great progress has been made. Copper catalysts
are widely used in this field for their economical and multifunc-
tional characteristics, especially for its less harmfulness to human
compared with other metals, such as chromium, cadmium, lead.
Now, it has been applied to the oxidation of alkanes [1–3], alkenes
[4], alcohols [5–8], arenes [9–11], benzylic substrates [12], and
other substrates [13] to corresponding aldehydes, ketones, epox-
ides and quinones with molecular oxygen [12], hydrogen peroxide
[14,15], tert-butyl hydroperoxide (TBHP) [16–19] and peroxyesters
[20–22] as oxidants. Many of these oxidation products are useful
and valuable intermediates for further synthesis of a wide variety
of compounds with industrial or medicinal value [23,24].
was obtained. Then in 1990, Small [32] improved the catalytic effi-
ciency with Cu(OH)2 as catalyst, but strict conditions such as high
pressure and temperature were needed. In 1998, Rothenberg et al.
[33] employed a copper(II) complex with n-Bu4N+Br− as ligand as
catalyst in the oxidation of tetralin with TBHP as oxidant and a con-
version of 50% was achieved. Late, several other copper complexes
[28,34] were reported in succession and their catalytic activities
in the oxidation of tetralin with TBHP as oxidant were discussed.
Though many complexes have been described in the literature for
the oxidation of tetralin, only few of them achieved results both
in high reaction rate and yield in mild conditions. Therefore, it
would make great sense to discover new complexes which can act
as efficient catalyst in the oxidation of tetralin to ␣-tetralone with
environmentally friendly oxidants.
Tetralone, an oxidation product of tetralin, is an important
intermediate for the production of dyes, pharmaceuticals and agro-
chemicals [25–27]. For example, it can be used in the synthesis
of carboryl, sertaline, 18-methyl norethisterone and a number of
medicinally useful derivatives [28,29]. Therefore, seeking for cata-
lyst with high performance for the oxidation of tetralin to tetralone
is meaningful and promising. Actually, a variety of homogeneous
and heterogeneous catalysts have been reported in literatures for
the oxidation of tetralin with environmentally friendly oxidants
For the purposes to seek highly efficient catalyst in the
preparation of tetralone from tetralin, herein, we report the syn-
thesis of two novel copper complexes 1 [Cu(AMP)Cl2] and 2
[Cu(AMP)2(H2O)2]Cl2 (AMP = 2-aminomethyl pyridine), and their
catalytic performances in the oxidation of tetralin towards ␣-
tetralone with TBHP as oxidant.
2. Experimental
2.1. Reagents
∗
Tetralin, 2-aminomethyl pyridine, urea hydrogen peroxide
(UHP) and tert-butyl hydroperoxide (TBHP) were purchased from
Corresponding author. Tel.: +86 22 60202926/60204279; fax: +86 22 60202926.
E-mail address: zhaojq@hebut.edu.cn (J. Zhao).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.10.014

174
C. Wang et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 173–179
Table 1
Alfa Aesar. All other reagents and solvents were obtained from com-
mercial sources and used as received without further purification.
Crystallographic data for the complexes.
Complex
1
2
2.2. Physical measurements
Empirical formula
Formula weight
Temperature (K)
Wave length (Å)
Crystal system
Space group
a (Å)
C6H8Cl2CuN2
242.58
113(2)
C12H20Cl2CuN4O2
386.76
143(2)
0.710 73
Monoclinic
P21/c
8.289 9(17)
7.749 1(15)
112.229(2)
92.11(3)
785.0(3)
2
IR spectra were recorded on a Bruker Vector-22 spectropho-
tometer using KBr pellets as the IR matrix. Elemental analyses
were performed on an Elementar Vario E1. Oxidation reactions
were monitored by a Shandong Lunan Ruihong gas chromatograph,
SP-6800A, equipped with an FID detector. The products were deter-
mined on a Thermo DSQ gas chromatograph–mass spectrometer.
0.710 73
Monoclinic
P2/c
14.427(3)
6.218 7(12)
19.113(4)
100.41(3)
1 686.5(6)
8
b (Å)
c (Å)
ˇ (◦)
V (Å3)
Z
2.3. Synthesis of the complexes
Dc (g cm−3
)
1.911
1.636
Size (mm)
Limiting indices
0.20 × 0.16 × 0.10
−17 < h < 15,
−7 < k < 7,
−22 < l < 22
3.154
968
2.17–25.02
11 507
2 959 (0.043 3)
2 008
2 959/0/199
1.082
R1 = 0.039 0,
wR2 = 0.093 5
R1 = 0.050 1,
wR2 = 0.099 7
0.630 and
−1.496
0.20 × 0.18 × 0.10
−10 < h < 10,
−10 < k < 7,
−15 < l < 16
1.740
398
3.60–27.87
6 113
1 844 (0.031 0)
1 590
1 844/3/109
1.124
R1 = 0.030 7,
wR2 = 0.083 0
R1 = 0.033 5,
wR2 = 0.083 7
0.449 and
−1.018
The complexes were synthesized based on the procedure
described by Detoni et al. [35] with some modifications. The syn-
thesis of complex 1 [Cu(AMP)Cl2] was carried out by the addition
of 1 equivalent of 2-aminomethyl pyridine in 2 ml of methanol to
1 equivalent of CuCl2·2H2O in 2 ml of methanol. The mixture was
then stirred for 30 min at room temperature and a large amount
of blue precipitate was formed. The precipitate was isolated by
filtration, washed with methanol and dried in vacuum.
Absorption coefficient (mm−1
F(0 0 0)
)
ꢁ range for data collection (◦)
Reflections collected
independent reflections (Rint
)
Reflections observed [I > 2ꢂ(I)]
Data/restraints/parameters
Goodness-of-fit (GOF)
The same procedure was conducted for the synthesis of complex
2 [Cu(AMP)2(H2O)2]Cl2 using 2 equivalents of the ligand.
Final R indices [I > 2ꢂ(I)]
(1) [Cu(AMP)Cl2]: blue solid. Yield about 82%. IR(KBr, disc, cm−1):
3288, 1602, 1481, 1433, 1287, 1162, 1094, 1038, 784. Elemen-
tal analysis calculated for C6H8Cl2CuN2 (242.58): C, 11.54; H,
29.69; N, 3.32%. Found: C, 11.53; H, 29.69; N, 3.31%.
(2) [Cu(AMP)2(H2O)2]Cl2: purple solid. Yield about 79%. IR(KBr,
disc, cm−1): 3211, 1604, 1485, 1434, 1373, 1291, 1144,
1031, 809. Elemental analysis calculated for C12H20Cl2CuN4O2
(386.76): C, 37.27; H, 5.21; N, 14.49%. Found: C, 37.26; H, 5.21;
N, 14.47%.
R indices (all data)
−3
˚
Largest diff. peak hole (e A
)
on a gas chromatograph–mass spectrometer. The conversion and
selectivity are calculated from the following formulas:
moles of tetralin consumed
moles of tetralin loaded
conversion (%) =
selectivity (%) =
× 100 ;
× 100
2.4. X-ray structure analysis
moles of ˛-tetralone
moles of tetralin consumed
Suitable crystal was obtained by slow solvent diffusion tech-
niques from methanol at 2–3 ◦C. Diffraction data of the complexes
1 and 2 were collected with a Bruker AXS APEX CCD diffractome-
ter equipped with a rotation anode using graphite-monochromated
3. Results and discussion
˚
Mo K␣ radiation (ꢀ = 0.71073 A). Structure solutions were found by
3.1. Synthesis of complexes
the Patterson method. Structure refinement was carried out by full-
matrix least-squares on F2 using SHELXL-97 with first isotropic and
later anisotropic displacement parameters for all non-hydrogen
atoms [36]. The H atoms were included in the calculation with-
out refinement. The crystal and instrumental parameters used in
the unit cell determination and data collection were summarized
in Table 1.
By changing the number of the ligands coordinated to the cen-
tral Cu(II) ion, several Cu(II) complexes containing 2,2ꢀ-bipyridyl
or 1,10-phenantroline ligands were synthesized by others previ-
ously [28]. Inspired by the results, herein, we use 2-aminomethyl
pyridine as ligand reacting with CuCl2·2H2O to synthesize the
corresponding Cu(II) complexes. Two mononuclear Cu(II) com-
plexes as shown in Fig. 1 were obtained smoothly by employing
1 or 2 equivalents of ligand to Cu(II). We also tried to prepared
the three ligands coordinated complex by dropping 1 equiva-
lent of CuCl2·2H2O or anhydrous CuCl2 to 3 equivalents or more
fold-excess of 2-aminomethyl pyridine in methanol. However, the
separated solid was confirmed to be complex 2 after characteriza-
tion not the target complex in any cases. The failure for the synthesis
of the three ligands coordinated complex is due to the large space
requirement of the complex and therefore high activation energy
is needed to overcome a great structure changes from complex 2,
as observed in a similar case by others [37].
2.5. General procedure for tetralin oxidation and analysis
The oxidation reactions were performed in a 25 ml round bot-
tomed flask equipped with a reflux condenser and immersed in
a water bath with temperature control. In a typical experiment,
0.045 mmol of the catalyst, 10 ml of acetonitrile and 4.5 mmol of
substrate were mixed in the flask. To the above mixture 22.5 mmol
of oxidant was added to start the reaction. The reaction mix-
ture analyses were carried out on a Shandong Lunan Ruihong
SP-6800A gas chromatograph equipped with a SE-30 column
(30 m × 0.25 mm internal diameter, 0.25 ␮m film thickness) and an
FID detector. The oven temperature programmed was 40 ◦C (hold
3 min), to 250 ◦C at 10 ◦C min−1 and hold for 5 min. Nitrogen was
used as carrier gas at 0.5 mL min−1 flow rate. The products were
identified by comparing with standard compounds and validated
3.2. X-ray crystal structures of complexes 1 and 2
The crystal structures of complexes 1 and 2 are shown in
Figs. 2 and 3, respectively, while selected bond lengths and angles

C. Wang et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 173–179
175
Table 2
2+ 2Cl-
Selected bond lengths (Å) and angles (◦) of complex 1.
[Cu(AMP)Cl2]
Cu(1)–N(1)
Cu(1)–N(2)
1.995(3)
2.004(2)
Cu(2)–N(3)
Cu(2)–N(4)
1.997(2)
2.002(2)
N
H2O
H2N
Cu(1)–Cl(2)
Cu(1)–Cl(1)
2.251 0(11)
2.272 0(10)
82.58(10)
97.08(7)
173.04(6)
170.99(7)
88.41(8)
Cu(2)–Cl(3)
Cu(2)–Cl(4)
2.250 7(11)
2.273 5(10)
82.71(10)
97.23(7)
174.87(6)
170.53(7)
87.99(8)
NH2
H2O
Cu
N
N(1)–Cu(1)–N(2)
N(1)–Cu(1)–Cl(2)
N(2)–Cu(1)–Cl(2)
N(1)–Cu(1)–Cl(1)
N(2)–Cu(1)–Cl(1)
Cl(2)–Cu(1)–Cl(1)
N(3)––Cu(2)–N(4)
N(3)–Cu(2)–Cl(3)
N(4)–Cu(2)–Cl(3)
N(3)–Cu(2)–Cl(4)
N(4)–Cu(2)–Cl(4)
Cl(3)–Cu(2)–Cl(4)
N
Cl
NH2
Cu
Cl
91.90(3)
91.89(3)
as a slightly distorted tetragonal pyramid. The basal plane is built
up by two nitrogen atoms N(1), N(2) provided by the chelating lig-
and and two chlorine atoms Cl(1), Cl(2), one being terminal and the
other acting as bridging atom. While the apical site is occupied by
the other bridging chloro ligand Cl(4).
[Cu(AMP)Cl2] (1)
[Cu(AMP)2(H2O)2]Cl2 (2)
Fig. 1. Schematic representation of the Cu(II) complexes.
˚
The distance of Cu(1)–Cl(1) is 2.2720(10) A slightly longer than
˚
that of Cu(1)–Cl(2) [2.2510(11) A]. Obviously the bridging role
of Cl(1) atoms lead to the slight lengthening of the Cu(1)–Cl(1)
bond distance compared with that of Cu(1)–Cl(2). The value of
˚
Cu(1)–Cl(4) distance is about 2.904 A indicating a very weak coor-
dination of Cl(4) to Cu(1). There are no distinct differences among
the Cu(1)–N and Cu(2)–N bond lengths, and the average Cu(1)–N
˚
and Cu(2)–N value is about 2.000 A which falls in a normal range
of Cu–N bond distance. The same Cu–N bond lengths also indi-
cates that N(2) or N(4) from the aminomethyl moiety has same
coordination capacity compared with that on the pyridine ring.
The Cl(1)–metal–basal ligand angles differ slightly from the
ideal value of a square pyramid (90◦) as the angle values of
N(2)–Cu(1)–Cl(1) and Cl(2)–Cu(1)–Cl(1) are 88.41◦ and 91.90◦,
respectively.
The crystal structure of 2 consists of two di-nitrogen chelating
ligands, two water molecules and two dissociated chlorine atoms.
In this structure, Cu(II) cation resides on a symmetry center and the
[Cu(AMP)2] part of the complex shows strictly planarity. A view
of the crystal structure along the crystallographic b-axis reveals
a layered arrangement of metal complexes. The plane on which
the metal cation locates forms octahedral geometry with the water
molecules at a semicoordination distance.
Fig. 2. Molecular structure of complex 1.
are listed in Tables 2 and 3. Crystals of 1 and 2 are both monoclinic
with space group P2/c and P21/c separately.
The crystal structure of 1, basically similar to that of
dimeric [{CuCl(PyTn)}2(␮-Cl)2] [38], consists of isolated binuclear
molecules unit with two bridging chloro ligands. But different from
that of [{CuCl(PyTn)}2(␮-Cl)2] with a symmetric crystal structure
unit, the crystal structure unit of 1 is unsymmetrical account for
a little difference in the corresponding bond lengths and angles
of each five-coordinated copper subunit (see Table 2). In the
structure unit of 1, two chloro ligands bridge the copper atoms
forming a four-membered ring, a terminal chloro ligand and a
bidentate chelating molecule of 2-aminomethyl pyridine complete
five-coordination at each metal. The bridging [Cu2Cl2] unit is pla-
nar by the presence of the crystallographic inversion centre in the
middle of the dimer. The geometry around Cu(1) can be described
The bond lengths of Cu–N in the basal plane are 2.0001(16) and
˚
2.0238(16) A, respectively, which have no great changes compared
˚
with those of complex 1. The axial Cu–O distances are about 2.539 A
showing a very weak coordination. The chloride ions are excluded
from the complexation form with a dissociated state.
3.3. General aspect of the oxidation of tetralin
The complex 1 as a catalyst was first applied to the partial oxida-
tion of tetralin with 65% TBHP as oxidant. The reaction proceeded
readily in high yield at 50 ◦C in acetonitrile and the reaction mixture
was analyzed by GC–MS. The results showed the main prod-
uct is ␣-tetralone, together with a major byproduct determined
as 1-tert-butylperoxytetralin and some other minor ones includ-
ing ␣-tetralol, naphthalene and 1,4-naphtoquinone (Scheme 1).
Table 3
Selected bond lengths (Å) and angles (◦) of complex 2.
[Cu(AMP)2(H2O)2]Cl2
Cu(1)–N(1)
Cu(1)–N(2)
N(2)–Cu(1)–N(2a)
N(2)–Cu(1)–N(1)
N(2)–Cu(1)–N(1a)
2.0238(16)
2.0001(16)
180.000(1)
83.16(7)
Cu(1)–N(1a)
Cu(1)–N(2a)
N(1)–Cu(1)–N(1a)
N(2a)–Cu(1)–N(1a)
N(2a)–Cu(1)–N(1)
2.0238(16)
2.0001(16)
180.000
83.16(7)
96.84(7)
96.84(7)
Fig. 3. Molecular structure of complex 2.

176
C. Wang et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 173–179
OOt-Bu
O
catalyst
+
TBHP
+
O
OH
t-BuOH
+
O
+
+
+
O
O
Scheme 1. Tetralin oxidation into different products.
The complex showed rather low selectivity to tetralol compared
with 1,10-phenantroline and bipy-Cu2+ complexes reported by
Atunes and colleagues [28]. The reason may be that 2-aminomethyl
pyridine has strong electron donation capacity compared with
1,10-phenantroline and bipyridine. The effect of electron donation
capacity of ligand on the electivity to ␣-tetralol was also observed
from the result that 1,10-phenantroline with lower electron dona-
tion capacity than bipyridine gave higher selectivity to tetralol in
the Cu2+ complex catalyzed oxidation of tetralin [28]. The combined
results support the idea that one can tailor the catalytic property
of the Cu(II) catalyst to either produce the hydroxylated product or
the ketone one. In addition, the radical of ␣-tetralol (TO•) derived
from the decomposition of 1-(tert-butylperoxy)-tetralin was more
prone to eliminating a hydrogen atom to generate ␣-tetralone than
capturing a hydrogen atom to form ␣-tetralol in the oxidation of
tetralin with TBHP as oxidant.
selectivity of ␣-tetralone. One explanation may be the stabilization
of Cu(I) species during the catalytic cycle caused by the coordina-
tion of acetonitrile molecules as in other oxidations reported by
others [39]. Acetone led to a lower activity than acetonitrile, which
may be due to the low-coordination of acetone molecules to the
metal centre of active Cu(I) species. Methylene dichloride has no
tendency to coordinate to metal therefore gave the lowest activ-
ity. On the contrary the protic solvent methanol can coordinate to
the copper ion strongly originating poorly active species, which led
to low reaction rate in the case. Otherwise, it is possible that the
oxidation of the methanol molecules took place, competing with
that of tetralin as in the case of copper catalyzed oxidation of other
substrates [40].
3.5. Effect of molar ratio of substrate to catalyst
Here, the products are derived from the pathways as shown in
Scheme 2.
The pathways clearly elucidates the formation of all the products
provided that 1-tert-butylperoxytetralin is the key intermediate
which is formed in the initial process as the mechanism proposed
by Sasson and colleagues [33] as shown in Scheme 3.
Four different catalyst loadings (0.009, 0.0225, 0.045
and 0.45 mmol) corresponding to the molar ratio of cat-
alyst/substrate/oxidant = 1/500/2500 (1), 2.5/500/2500 (2),
5/500/2500 (3) and 10/500/2500 (4) were chosen to study
the effect of molar ratio of substrate to catalyst on the reaction.
The results were shown in Fig. 4.
As can be seen from Fig. 4, that the higher the molar ratio of
catalyst/substrate/oxidation was, the more product was obtained
at the same reaction stage most of the time. In other words, the
reaction depended on the concentration of catalyst. It was also
observed that the reaction under the catalyst/substrate ratios of
5/500 and 10/500 reached the same value being the highest yield of
␣-tetralone received when the reaction proceed for 11 h. Actually,
the curves representing the yield variation with time in the above
two cases already overlapped after the reaction proceeded for about
8 h, which indicated the presence of decomposition of overloaded
catalyst in the catalytic run especially in the later period. After
reached its maximum the yield decreased gradually with reaction
time, which was probably resulted from the ␣-tetralone’s further
oxidation to 1,4-naphthalenedione as shown in Scheme 2.
3.4. Effect of the solvent
Oxidation of tetralin can be carried out in different solvent in
literature [28,33,34]. The initial conditions studied for this reaction
were based on previous tetralin oxidation studies [28]. For select-
ing a suitable solvent for the oxidation of tetralin using complex
1 as catalyst and TBHP as oxidant, the reactions were carried out
in methanol, acetonitrile, acetone and dichloromethane at 30 ◦C,
respectively, considering their low boiling point and volatility. The
results are given in Table 4.
Generally speaking aprotic polar solvents such as acetonitrile
and acetone are superior to low polar solvent methylene chlo-
ride and protic solvent methanol. Moreover, acetonitrile is the best
among the selected solvents both in conversion of tetralin and
Table 4
Oxidation reaction of tetralin in different kinds of solvents using complex 1 as catalyst.
Catalyst
Solvent
Time (h)
Conversion (%)
Selectivity (%)a
CH3OH
CH3CN
CH3COCH3
CH2Cl2
10
1
1.5
5
22
22
22
11b
70
75
75
71b
[Cu(AMP)Cl2]
Reaction conditions: solvent = 10 ml, [Cu(AMP)Cl2] = 0.045 mol, tetralin = 4.5 mmol, 65% TBHP = 22.5 mmol, T = 30 ◦C.
a
Selectivity of ␣-tetralone.
After 5 h, the reaction reached its end-point.
b

C. Wang et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 173–179
177
t-Bu
O O
CuII-OO-t-Bu
+
OH
O
OH
CuII-OO-t-Bu
O-t-Bu
O
CuII-OO-t-Bu
O
OH
OH
O-t-Bu
O
O
O
OH
O
.
2BuO
OO
Scheme 2. The pathways for the generation of the products.
3.6. Effect of temperature
50
40
30
20
10
Four different temperatures (15, 30, 50 and 80 ◦C) were chosen
to study their effect on the oxidation of tetralin with complex 1
as catalyst, and both the selectivity of ␣-tetralone as well as the
time needed to reaching the maximum conversion of tetralin were
monitored. The results are listed in Table 5. The results show that
higher reaction temperatures are in favor of the tetralin oxidation
both in conversion rate and selectivity of ␣-tetralone in the range
of 15–50 ◦C. At 15 ◦C, 32 h were needed to reach the conversion of
88% with a selectivity of only 43% to ␣-tetralone, while at a reaction
temperature of 50 ◦C, the same conversion was achieved in less
than 1 h but with a selectivity of 71%. However, the conversion of
tetralin at a reaction temperature of 80 ◦C decreased in some degree
compared with that at 50 ◦C.
(1)
(2)
(3)
(4)
.
CuI + t-BuOO
+ H+
.
CuII + TBHP
CuI + TBHP
(1)
(2)
(3)
CuIIOH + t-BuO
0
2
4
6
8
10
12
14
CuII-OO-t-Bu + H2O
CuIIOH + TBHP
.
Reaction time(h
)
.
Fig. 4. Effect of the molar ratio of substrate to catalyst on the reaction using complex
1 as catalyst.
(4)
(5)
t-BuOH + R
RH
t-BuO +
.
R-OO-t-Bu + CuI
CuII-OO-t-Bu + R
Scheme 3. The mechanism for the formation of 1-tert-butylperoxytetralin.

178
C. Wang et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 173–179
Table 5
Effect of the temperature on the oxidation of tetralin using complex 1 as catalyst.
Catalyst
Temperature (◦C)
Time (h)
Conversion (%)
Selective (%)
15
30
50
80
32
10
1
88
88
89
87
43
54
71
71
[Cu(AMP)]Cl2
1
Reaction conditions: acetonitrile = 10 ml, [Cu(AMP)Cl2] = 0.045 mmol, tetralin = 4.5 mmol, 65% TBHP = 22.5 mmol.
For elucidating the reasons leading to these results, the reac-
tion mixtures at different temperatures were analyzed by GC–MS
and it was found that higher content of tetralin peroxide was
detected at low temperature than that at high temperature. There-
fore, the results listed in Table 5 can be explained as that the
radical can be generated at mild temperature and then reacts
with tetralin to give 1-tert-butylperoxytetralin. Obviously, ele-
vating reaction temperature can accelerate the decomposition of
1-tert-butylperoxytetralin to ␣-tetralone between 15 ◦C and 50 ◦C,
therefore, increase the reaction rate as well as the selectivity of
␣-tetralone. With the temperature elevating higher and higher,
for example 80 ◦C, the radical and TBHP might decomposed to
other inactive species, which led to the decrease of conversion
of tetralin slightly. Besides, high temperature might also lead to
the decomposition of catalyst as the cases using Co as catalyst
[41].
3.8. Catalytic evaluation of complexes 1 and 2
Complex 2 was also used as catalyst in oxidation reaction under
conditions optimized for complex 1 with TBHP as oxidant and the
results are shown in Fig. 6. It can be seen that complex 2 showed
low activity in the reaction compared to complex 1. It is note-
worthy that the conversion with complex 2 as catalyst gradually
increased with the reaction time, no conversion jump was observed
in the whole course. Therefore, it can be concluded that complex
2 as a whole acted as catalyst to catalyze the reaction advanc-
ing, no ligand disassociation taking place. Recalling the structure
difference between complex 1 and complex 2, it is not too hard
to understand why the later is less active than the former one.
In complex 2 two AMP are coordinated to Cu(II) tightly, which
induces a strong steric hindrance preventing the approaching of
TBHP to Cu(II) and then slows down the formation of active species
to the oxidation reaction. In the other hand, from the structure
it also can be seen that the electron density of the central Cu(II)
of complex 2 is higher than that of complex 1, which is disad-
vantageous to the access of TBHP as a nucleophilic reagent to the
metal center and, consequently, brings down the oxidation effi-
ciency.
Though the differences in the catalytic activity are present
between complexes 1 and 2, same selectivity ranging from 60%
to 75% towards ␣-tetralone was observed under the same conver-
sion (different reaction time needed) in the two cases. This can be
explained by the fact that the generation of the ligand-involved
intermediates to give byproducts needs higher active energy than
that of the 1-tert-butylperoxytetralin leading to ␣-tetralone. This
phenomenon was also observed in the other copper complexes
with a same ligand catalyzed oxidation of tetralin [28,34]. Navarro
et al. [34] attributed it to the complexes with the same lig-
and.
3.7. Comparison of different oxidants
Besides 65% TBHP, 30% H2O2 and UHP were also used as oxidants
in the oxidation of tetralin with complex 1 as catalyst. The reactions
were carried out in the optimized conditions discussed above and
the results are shown in Fig. 5.
Compared to TBHP, 30% H2O2 and UHP both performed poorly
in the reaction from the point of view of conversion of tetralin.
In the case of TBHP as oxidant the reaction completed within 4 h,
however, only conversions of 23% and 36% were obtained when
using UHP and 30% H2O2 as oxidants in the same conditions,
respectively. In addition, extending reaction time did not receive
prominent increase of the conversion. However, the selectivity
towards ␣-tetralone was higher with 30% H2O2 and UHP as oxi-
dants due to their low oxidability preventing the further oxidation
of ␣-tetralone.
100
80
100
(1)
(2)
(3)
80
60
40
20
0
(4)
(5)
(6)
60
40
(1)
(2)
(3)
(4)
20
0
0
1
2
3
4
5
6
7
8
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17
Time(h)
Time(h)
Fig. 5. Comparison results of different oxidants on reactions using complex 1 as
catalyst.
Fig. 6. Conversion and selectivity catalyzed by complexes 1 and 2.

C. Wang et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 173–179
179
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Two mononuclear Cu(II) complexes [Cu(AMP)Cl2] and
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